1. Field of the Invention
The present invention relates to a method of correcting scanning rate, or speed of a galvanometer, and to an apparatus for correcting non-linear scanning rate, or speed of the galvanometer that is employed in an optical information recording apparatus, or reading apparatus.
2. Description of the Prior Art
FIG. 1 illustrates a conventional information recording apparatus a computer output microfilm apparatus (referred to as "laser-COM" hereinafter), in which print data (variable information) supplied from the computer and desirable form data (fixed information) are recorded on microforms using a laser beam as scanning light.
With reference to FIG. 1, an argon (Ar) laser 1 emits blue green light beams for recording purposes. The blue green light beams B are intensity-modulated in an optical modulator 2 by video signals (will be discussed later) and thereafter pass through a first dichroic mirror 3. A helium-neon (He - Ne) laser 4 emits red light beams R for reading purposes, which are denoted by "R". The red light beams R are incident upon a first reflecting mirror 5 and reflected therefrom to a first dichroic mirror 3. The red light beams R are reflected from the first dichroic mirror 3 and mixed with the other light beams for recording purposes that have passed through this dichroic mirror 3. The combined light beams are incident on a rotating polyhedric mirror 7 through a second reflecting mirror 6. In this case, the first dichroic mirror 3 is designed to pass the blue and green light beams therethrough and to reflect the red light beams thereon.
The rotating polyhedric mirror 7 is rotated in a predetermined direction at a constant rate by a motor 9 to which power is supplied from a motor drive circuit 8. As a result, the combined light beams R, B incident upon the respective mirror surface of the rotating polyhedric mirror 7 are reflected from these mirror surfaces and simultaneously deflected (referred to as "horizontal-deflected beams"). Then, the mixed light beams are converted into primary scanning light having a repetition period that is defined by the beam reflections occurring from the respective mirror surfaces of the rotating polyhedric mirror 7. The primary scanning light is incident upon a second dichroic mirror 11 via a convergent optical system 10. The second dichroic mirror has such characteristics that the recording blue-green light beams and the reading red light beams can be transmitted therethrough and a part of the reading red light beams can be reflected thereon. Accordingly, in the mixed light beams incident upon the second dichroic mirror 11, both the blue-green light beams B and the red light beams R pass toward a galvanometer 12, and the red light beams R are partially reflected and incident upon a linear encoder 13.
In response to saw-tooth driving signals supplied from a galvanometer driver 14, the galvanometer 12 deflects the recording light beams R, B in a direction substantially perpendicular to the horizontal deflecting direction (referred to as "vertical deflection"). As described above, the galvanometer driver 14 produces the saw-tooth driving signals based upon clock signals derived from a clock signal generator 15 (which will be discussed later). For instance, counting these clock signals in a vertical address signal generator 16 in the vertical deflection period enables the vertical address signals to be produced. In response to these address signals, the galvanometer driver 14 produces the above-described saw-tooth driving signals.
Since the blue green light beams and also the red light beams vertically deflected by the galvanometer 12 have been converted into the one dimensional scanning light by the rotating polyhedric mirror 7, they become two dimensional scanning light by means of such vertical deflections. Then, the two dimensional scanning light as incident upon a third dichroic mirror 17, thereby splitting it into the blue green light and the red light.
The two dimensional scanning light of the blue green light beams passing through the third dichroic mirror 17 is focused on recording materials such as films via a focusing optical system 18 to raster-scan them. The other two dimensional scanning lights of the red light beams split by the third dichroic mirror 17 is incident upon a form slide film 20A via a third reflecting mirror 19.
In a form slide film device 20, a plurality of form slide films 20A, 20B, - - - , 20N are present as needed. Different slide images and writing frames constituted by a plurality of vertical and horizontal lines are recorded on these slide films 20A, 20B, - - - , 20N. For the sake of simplicity, only two form slide films 20A and 20B are illustrated. One of these form slide films is selectively moved to a scanning position where it is scanned by the above two dimensional scanning light. As desired, the form slide films 20A, 20B, - - - , 20N are arbitrarily detachable from the form slide device 20.
As seen from FIG. 1, the two dimensional scanning light R passes through the form slide film 20A and is converted in a first photomultiplier 21 to electric readout signals. The readout signals correspond to video signals of the writing frame image of the scanned form slide film 20A.
The red light beams R split by the second dichroic mirror 11 are, on the other hand, incident upon a linear encoder 13 to be one-dimensional-scanned. The linear encoder 13 is formed by a plurality of transparent and non-transparent line-shaped grids which are aligned parallel to the horizontal deflection direction and equidistantly separated to form a straight striped pattern. Pulsatory light obtained by scanning this linear encoder 13 by means of the horizontal deflection scanning light is converted by a second photomultiplier 22 into pulse signals as clock pulse signals. By applying these clock pulse signals to a phase-coupling type clock signal oscillator 23, clock signals are oscillated. The clock signals are used to synchronize the respective circuit elements of the laser-COM with each other. The linear encoder 13, second photomultiplier 22, and clock signal oscillator 23 constitute the clock signal generating device 15.
Under the timing control of the clock signals derived from the clock signal generating device 15, character information corresponding to coded data from the character information source such as magnetic tapes etc. can be read out from a character generator 24 as video signals. These video signals derived from the character generator 24 are supplied to a signal composite circuit 25. While the form signals that are obtained by amplifying outputs of the first photomultiplier 21 in the amplifier 26 and thereafter shaping them in a level slicer 27 are supplied to the signal composite circuit 25, the above video signals are combined with the form signals in the signal composite circuit 25.
Thus the composite video signals are supplied through a modulator drive circuit 28 to the optical modulator 2 so as to intensity-modulate the recording light beams. As easily seen, the raster-scanned image projected toward the film F corresponds to an image formed from print data derived from the computer and written in a given position of the form frame selected by the form slide film.
Such an information recording apparatus is known from, e.g., U.S. Pat. Nos. 4,323,906 and 4,340,894.
As previously described in detail, the raster scanning in the optical scanning type information recording apparatus is accomplished by the horizontal scanning of the rotating polyhedric mirror 7 and also the vertical scanning of the mirror of the galvanometer 12.
Referring to FIG. 2, the arrangement of the galvanometer 12 will now be described in detail. A mirror 12c mounted on a shaft 12b of a galvanometer motor 12a is repeatedly pivoted within a predetermined rotating angle. The scanning light beam coming from the rotating polyhedric mirror 7 is reflected thereon and simultaneously deflected in the vertical deflection direction as denoted by an arrow V, thereby directing the deflected scanning beam toward the film F. It should be noted that the third dichroic mirror 17 is omitted in FIG. 2.
The galvanometer 12 is driven by input drive signals which are supplied from the galvanomirror driver 14. The drive signals vary in a linear mode as shown in FIG. 3A. Due to the inherent characteristics of the galvanometer 12, the rotating speed of the motor shaft 12b, i.e., the scanning speed in the vertical deflection direction is not constant, thereby deteriorating the linearity of the galvanometer 12. As a result, the recorded images on the film F are distorted. Such distortions can be reproduced in that the motor 12a of the galvanometer 12 is driven by an input drive signal varying in a non-linear mode as shown by a solid line in FIG. 3B.
Accordingly, the linearity of the galvanometer 12 is improved by the servo system in the galvanometer driver. However, satisfactory results cannot be obtained in the conventional system.
It is therefore understood that considering the non-linear characteristic of the scanning speed of the galvanometer, an amount of deviation between the scanning speed and the desirable constant scanning speed (i.e., linearity) must be corrected.
Specifically as to the point of resolution, when, for instance, the microfilm image having a size of 9 mm.times.12 mm is enlarged into a A4-sized image, the desirable resolution of approximately 200 rasters/mm on the microfilm image is required so as to reproduce the resolution of 8 rasters/mm on the enlarged image. Accordingly, since very small distortion is emphasized in the enlarged image in the laser-COM, the linearity of the scanning speed of the galvanometer is a very important factor so that a precisely controlled scanning speed in absolutely required in such a laser-COM.
If the input drive signal represents such a non-linear characteristics as shown in FIG. 3B, an input drive signal having another non-linear characteristic opposite to that of FIG. 3B must be used to drive the galvanometer 12 (see FIG. 3C).
An object of the present invention is to provide a method of correcting the non-linear scanning speed of the galvanometer to derive the linear scanning speed.
A further object of the invention is to provide an apparatus for correcting the non-linear scanning speed of the galvanometer.